The British police investigation into Madeleine McCann's disappearance moved a step closer to making arrests yesterday after Portuguese authorities received a letter of formal request for help.

Portugal's Attorney General's office confirmed they were in receipt of an International Letter of Request - a "significant development," according to a source close to Madeleine's parents Kate and Gerry McCann.

The contents of the letter are believed to ask for access to three prime suspects.

The Mirror has previously reported how British police are braced to make the first arrests ever in case of Madeleine McCann's disappearance.

The letter will now be forwarded to a court in the Algarve resort of Portimao tasked with investigating the newly-reopened case in Portugal.

Authorities declined to give a timescale on when the letter would be sent or actioned.

Scotland Yard are understood to want to question three local burglars blamed for a string of break-ins in Praia da Luz where Madeleine was snatched in May 2007.

Portuguese detectives acting on a first International Letter of Request sent by Britain last July discovered the men were very close to the scene of Madeleine's disappearance.

They are thought to have made an unusually high number of calls to each other in the hours after her abduction.

Portugal's Attorney General's office announced last October the Madeleine McCann inquiry was being reopened at the request of police in Porto who have spent the past three years reviewing the case.

Jose Magalhaes e Menezes, the public prosecutor in charge of the original investigation, is overseeing the reopened inquiry.

He was damning of the original probe pointing the finger at Madeleine’s parents Kate and Gerry and claimed officers under disgraced former police chief Goncalo Amaral achieved "very little" during their 15 month investigation.

Scotland Yard detectives are conducting their own separate criminal inquiry into Madeleine's disappearance called Operation Grange.

Detective Chief Inspector Andy Redwood, who is leading the Met operation, has said he continues to believe Madeleine is still alive.

Operation Grange was first launched as a review in 2011 following the intervention of David Cameron in the case.

A source close to Maddie's parents said: "It it the next step forward in what is proving to be a slow but very thorough process.

"Met Police want to be given the opportunity to arrest key suspects.

"They are keeping Kate and Gerry informed of all developments.

"We understand the International Letter of Request was received by the Portuguese authorities last week.

"It is an important and significant development.

"But it is too early to say, at this stage, if the powers that be in Portugal will co-operate."

The McCann's official spokesman Clarence Mitchell said last night: "Kate and Gerry do not comment on any aspect of Operation Grange."